1. Field of the Invention
This invention relates to a muffler hanger of an automobile, that is, an exhaust pipe supporting device to support an exhaust pipe by a vehicle body in a vibration-proof manner.
2. Description of Related Art
Heretofore, a supporting device integrally molded to an exhaust pipe consisting of a rubber elastomer to support an exhaust pipe to a vehicle body in a vibration-proof manner has been known, in which a first holding portion having a through-hole in which a supporting member extending from the vehicle body is inserted, and a second holding portion having a through-hole in which a supporting member extending from the exhaust pipe side is inserted, are connected by two right and left arch-shaped side wall portions so as to make both through-holes be in parallel and also oppose each other across a vertical gap, the shape of the supporting device being generally annular.
Although this supporting device elastomer absorbs vibration, right and left side wall portions are prone to vibrating due to shaking by the vehicle body or the exhaust pipe, and dynamic absolute spring rate rises in the high frequency region (for example, a region of 500 Hz or more). Since one cannot expect adequate vibration transmission suppression, it has been suggested to connect right and left both side wall portions by disposing a center connecting portion at intermediate position between the first and second holding portions (JP-A-66953/1991).
With regard to the suggestion described above that both side wall portions are simply connected by the center connecting portion, there may be the cases where adequate effect of vibration transmission suppression cannot be accomplished, for example, in a range of 300 to 400 Hz or thereabout bandwidth, due to discrepancy of frequency characteristics when atmospheric temperature rises. Accordingly, in order to improve upon this, an applicant suggests a supporting device provided with a center vibration damping portion, in which, instead of the center connecting portion described above, there is a center portion, that is, a mass portion, the mass of which is greater than that at both side portions, and both side portions split midway into two and are connected respectively with the right and left side wall portions (JP-A-264381/1997).
As described above, the supporting device, in which the right and left side wall portions are connected at the center vibration damping portion having the mass portion at the center portion, has normally three vibration modes, or absolute spring rate peaks; for example, a first mode at close to 250 Hz, a second mode at close to 500 Hz, and a third mode at close to 1000 Hz.
These three vibration modes are caused by the following vibrations, respectively, according to analysis of the present inventors. The first mode is ascribed to vibrational resonance induced with so-called xe2x80x9cmass-spring systemxe2x80x9d vibration as its major factor: the mass portion vibrates vertically, with the mass portion at the center vibration damping portion taken as xe2x80x9cmassxe2x80x9d and right and left connecting portions which support the mass portion on both sides thereof taken as xe2x80x9cspringsxe2x80x9d. The second mode is ascribed to vibrational resonance of flexural vibration of the mass portion (flexural vibration of the mass portion is considered as a xe2x80x9cbeamxe2x80x9d with front and rear ends thereof being free to move) with vertical vibration of both sidewall portions which together form a ring. The third mode is ascribed to resonance of respective vibrations of the two side wall portions which together form a ring.
Incidentally, there are the cases where in loading the exhaust pipe supporting device it may be required to make absolute spring rate small in a specific frequency range for a specific vehicle. In that case, when there exists one of the vibration modes described above in this specific frequency range, it is difficult to satisfy this requirement adequately only by lowering the peak value of the absolute spring rate in the vibration mode. Accordingly, it is desirable to reduce the absolute spring rate in the specific frequency range by shifting the resonance frequency of the vibration mode away from the specific frequency range.
In view of the aforementioned problems and difficulties, the present invention has an object to provide an exhaust pipe supporting device having a center vibration damping portion which can shift the resonance frequency in the vibration mode to a lower frequency and thereby reduce the absolute spring rate in a special frequency range.
An exhaust pipe supporting device of the present invention is integrally molded with the exhaust pipe, consists of a rubber elastomer, and has a first holding portion having a through-hole in which a supporting member extending from the vehicle body side is inserted, a second holding portion having a through-hole in which a supporting member extending from the exhaust pipe is inserted, a pair of side wall portions which connect on both the first holding portion and the second holding portion disposed at intervals, and a center vibration damping portion interposed at an intermediate position between the first and the second holding portions and connected to both side wall portions, wherein the center vibration damping portion comprises a center portion formed as a mass portion and a pair of connecting portions which connect both sides of the mass portion to the side wall portions, the mass portion is formed to have larger mass than that of the connecting portion, the width (T1) of the connecting portion in the longitudinal direction of the supporting device is set smaller than the length (T2) of the side wall portion in the longitudinal direction of the supporting device. Now, the longitudinal direction of the supporting device is the longitudinal direction of the device, normally the axial direction of the through-holes.
In reference to the supporting device of the invention, in order to set the size (T1) of the connecting portion smaller than the size (T2) of the side wall portion, the center vibration damping portion may be provided with a concave portion depressed in the longitudinal direction at the connecting portion relative to the side wall portion and the mass portion on both sides thereof.
In this case, the concave portion may be provided on either the front or the back side or on both sides in the longitudinal direction of each of the connecting portions on both sides of the mass portion. In the former case, furthermore, on the connecting portions on both sides of the mass portion, the concave portions may be provided both on the front, both on the back, or one each.
In reference to the supporting device of the invention, it is preferable that the width (T1) of the connecting portion is set to 20 to 90% of the length (T2) of the side wall portion.
In reference to the supporting device of the invention, the size (T3) of the mass portion in the longitudinal direction may be set larger than the length (T2) of the side wall portion, and thereby the mass of the mass portion can be increased to realize superior vibration-damping function.